The lungs are the primary organs of the respiratory system. The main function of the human respiratory system is to transport oxygen from the atmosphere into the blood, and to expel carbon dioxide from the body. Healthy levels of oxygen are absolutely crucial for the human body, as oxygen gives our cells energy and helps them regenerate.Each lung is divided into lobes. The right lung, which has three lobes, is slightly larger than the left, which has two. The lungs are housed in the chest cavity, or thoracic cavity, and covered by a protective membrane called the pleura. The diaphragm, the primary muscle involved in respiration, separates the lungs from the abdominal cavity.